The ability to vaccinate an at-risk civilian population with vaccinia virus is central to preparing for the potential threat of smallpox bioterrorism. However, a critical limitation of this strategy is the recognized complications of vaccinia vaccination, particularly in immunocompromised hosts, pregnant women, and infants. Therapeutic interventions currently available to counter such complications are inadequate and novel strategies are needed. We propose to develop such new therapies that target related, yet functionally distinct, vaccinia proteins. The vaccinia complement-control protein (VCP) and the extracellular enveloped virus (EEV)-specific B5R protein both contain short consensus repeat (SCR) units present in complement regulatory proteins. We have shown that VCP and the B5R proteins are critical for pathogenesis in vivo. VCP inhibits complement activation and helps the virus evade the host complement mediated attack. The B5R protein is essential for efficient viral dissemination. Our hypothesis is that these viral SCR-containing proteins have critical functions in pathogenesis that make them uniquely suited to serve as novel targets for therapeutic strategies directed at complications occurring during vaccinia immunization. Individuals with life-threatening vaccinia virus vaccine complications usually have defective cell-mediated or humoral immunity, but typically have intact innate immune function. A therapeutic inhibitor of VCP's complement control activity would therefore be a novel approach to managing vaccinia vaccine complications because such an inhibitor would allow the host's innate immune system to regain control of the infection. In Specific Aim number 1, we will utilize phage library display to identify specific inhibitors of VCP that prevent its inhibition of the complement cascade. The B5R protein is one of several EEV-specific proteins. B5R also contains SCRs and, while complement regulatory activity has not been identified, we and others have demonstrated that B5R is critical for EEV formation and viral spread in vivo. In addition, recent reports have shown that B5R is one of the principal targets for EEV neutralizing antibodies. Thus, therapeutic targeting of B5R offers an additional way of controlling vaccinia virus replication and dissemination. In Specific Aim number 2 we will develop monoclonal antibodies (mAbs) to the B5R protein and identify mAbs that neutralize EEV. We believe that identification of such mAbs (along with a cocktail of humanized mAbs to other EEV-specific proteins) can form the basis for a passive immune neutralization strategy to control vaccinia virus vaccine complications. We anticipate that these proteins will provide novel targets for immunomodulation of vaccinia virus. In addition, because both VCP and BSR are present in variola virus, these new therapies may be effective against smallpox infection.